In general, there are two types of lifting assembly (referred to as "hardware"), used to lift overhead sectional doors.
The first type is known as a torsion spring system. It involves a single steel shaft mounted to the header of the door opening. A pair of torsion springs are slipped over the ends of the shaft and abut a fixed central anchor bracket at their inner ends. The free end of each spring is then connected with a winding plug slipped onto the shaft. A winding rod is used to turn the plug and wind the spring. The plug is then secured to the shaft with set screws. A cable drum is mounted at each end of the shaft. A cable is secured to each drum and extends down to the base of the door, to which it is attached. The door is pulled down and locked in place. When the door is unlocked, the springs unwind, turn the shaft and roll the cables up on the drums, thereby lifting the door.
The beauty of the torsion spring system is that the two cables are wound up by a common shaft. The rate of winding for each cable is therefore the same. As a result, the door rises smoothly--it does not become canted and bind as it is raised.
A disadvantage of the torsion spring system is that the hardware is relatively expensive.
An extension spring system commonly involves mounting two extension springs, one on each side of the door. Each spring is anchored at its base to the lower end of the door opening frame. A movable pulley is attached to the upper end of the spring. A second fixed pulley is suspended from a head plate secured to the door opening frame. A cable is anchored at one end to the head plate. Its free end passes around the pulleys, extends down the length of the door and is attached at its lower end to the door. When the door latch is released with the door closed, the two springs contract and each spring pulley pulls up on its cable, which causes the door to be raised.
One problem with the extension spring system is that the springs may pull unevenly, causing the door to cant and bind. However, this system is less expensive than the torsion spring system and it is therefore commonly used for household garage doors.
Another problem with the extension spring system has to do with the usual technique used to attach the spring to the door frame. This technique involves first lifting the door to the "up" position and blocking it there. The spring is then attached to the door frame after which the door is unblocked and lowered to stretch the spring. Raising the dead weight of the door usually requires more than one person. Installers are therefore not favorably inclined to extension spring systems because of the need for more than one person to carry out installation.
An objective of the present invention is to provide an extension spring-type system which, like a torsion spring system, applies equal lifting force and raising rate to each side of the door, to thereby achieve a smooth lifting action free of canting and binding, while still maintaining the lower cost associated with an extension spring assembly.
Another objective is to facilitate installation of the system using only one person.